Added scheduler selenium spec to cover a regression test case

Fixes CNVS-17529

Test Plan:
* Run /spec/selenium/teacher_schedule_spec.rb and make sure everything passes

Test case: https://canvas.testrail.com/index.php?/cases/view/85934&group_by=cases:section_id&group_order=asc&group_id=4118

Change-Id: I9a6087a1326bffa23ee7ae886371448d58f89469
Reviewed-on: https://gerrit.instructure.com/45934
Reviewed-by: Sean Lewis <slewis@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Product-Review: Derek Hansen <dhansen@instructure.com>
QA-Review: Derek Hansen <dhansen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
allow enrollments in multiple sections via Add People dialog

(note that the Enrollments API already works this way,
and the Edit Sections dialog can be used to accomplish this
also)

test plan:
 0. have a course with multiple sections
 1. have a user with a login
 2. use the Add People dialog to enroll the user in the course
 3. use the Add People dialog again, to enroll the same user
    with the same enrollment type as before, but choose a
    different section from the dropdown
 4. the user should be enrolled in both sections

fixes CNVS-17594

Change-Id: Ia39537591c779af9ba3fc3163a32d6487c2c4e88
Reviewed-on: https://gerrit.instructure.com/46165
Reviewed-by: Jon Willesen <jonw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>